The invention relates to a metal halide lamp provided with a discharge vessel with a ceramic wall which encloses a discharge space in which an ionizable filling is present, two electrodes having tips with a mutual distance EA being arranged in said discharge space, the latter having an internal diameter Di at least over the distance EA.
A lamp of the kind mentioned in the opening paragraph is known from EP-A-0 215 524. The known lamp, in which a high luminous efficacy goes hand in hand with excellent color properties (inter alia a general color rendering index R.sub.a .gtoreq.80 and a color temperature Tc of between 2600 and 4000 K), is highly suitable as a light source for inter alia interior lighting. This lamp construction is based on the recognition that a good color rendering is possible when sodium halide is used as a filling ingredient of a lamp and a strong widening and inversion of the Na emission in the Na-D lines takes place during lamp operation. This requires a high coldest-spot temperature T.sub.kp in the discharge vessel of, for example, 1170 K (900.degree. C.). Inversion and widening of the Na-D lines imply that they take the shape of an emission band in the spectrum with two maxima at a mutual distance of .DELTA..lambda.. The requirement that T.sub.kp should have a high value excludes under practical conditions the use of quartz or quartz glass for the discharge vessel wall and renders the use of a ceramic material for the discharge vessel wall necessary.
The term "ceramic wall" in the present description and claims is understood to cover a wall of metal oxide such as, for example, sapphire or densely sintered polycrystalline Al.sub.2 O.sub.3 as well as metal nitride, for example AlN.
The known lamp combines a good color rendering with a comparatively wide range of the color temperature. The filling of the discharge vessel comprises at least Na halide and Tl halide. In addition, the discharge vessel preferably contains at least one element from the group formed by Sc, La, and the lanthanides Dy, Tm, Ho, and Er. The known lamp has a comparatively short discharge vessel for which it is true that 0.9.ltoreq.EA/Di.ltoreq.2.2, and a high wall load which is more than 50 W/cm.sup.2 for practical lamps. The wall load is defined here as the quotient of the lamp power and the outer surface of that portion of the discharge vessel wall which is situated between the electrode tips.
It is a disadvantage of the known lamp that it has a comparatively limited luminous efficacy for general lighting purposes.
U.S. Pat. No. 4,972,120 discloses a lamp which radiates white light with reasonable color properties (3000 K.ltoreq.T.sub.c .ltoreq.4000 K; R.sub.a approximately 50-60) and which has a comparatively high luminous efficacy. This lamp, however, requires a solenoidal electric field for energizing the discharge, for which purpose the lamp is provided with an external coil which is wound largely around the discharge vessel. The coil is to be operated at a very high frequency of more than 1 MHz. Although the light radiated by the lamp is in itself quite useful for general lighting purposes, the exceptional construction of the lamp and the specific electric supply equipment required for it render the use of this lamp for general lighting purposes not very practical.
U.S. Pat. No. 3,786,297 describes discharge lamps having very high luminous efficacies and provided with electrodes. The filling of the discharge vessel for this purpose comprises at least Cs halide and a comparatively large quantity of Hg (between approximately 3 mg/cm.sup.3 and 20 mg/cm.sup.3) which has a pressure of more than 3 at during lamp operation. Although Cs has a low ionization voltage, radiation from Cs lies for a considerable portion outside the visible part of the spectrum. It was found that the light radiated by the lamp has color properties such that it is less suitable for use in general lighting. The use of a large dose of Hg is undesirable for environmental reasons.
An important disadvantage of the metal halide lamps fitted with electrodes and having a high luminous efficacy is the major risk of spiraling instabilities occurring in the discharge, and of additive segregation in the filling of the discharge vessel.